Editor’s Note: The following article is excerpted from Joe Kissell’s Take Control of Mac OS X Backups, Fourth Edition. It includes two portions of a chapter about backup media: the first portion looks at backing up to a local, external drive and the second has Joe’s overall recommendations for backup media. Take Control of Mac OS X Backups, Fourth Edition is a 196-page electronic book that explains how to create a backup strategy that protects your data and enables quick recovery in case of catastrophe. It is available for $15 from TidBits Publishing.

When it comes to safeguarding your data, I use and suggest external hard drives as a backup medium. In almost every case, I believe they’re the best choice for individuals and small networks.

I deliberately said “external hard drives”—even though you could save some money on the enclosures and extra electronics by buying bare drives that can be mounted inside your desktop Mac Pro or Power Mac. I advocate external drives because:

You can disconnect an external drive and store it off-site—an important safeguard against theft.

If your computer suffers severe damage due to a power surge, a leaky roof, or being knocked off the desk accidentally, your internal hard drives may fail along with the rest of the machine.

Hard drive virtues

You may feel anxiety about the cost of external hard drives—especially since you should have at least two, and perhaps three of them. They may seem extravagant in a way that DVD-Rs, say, do not. So let me sing the praises of hard drives for a moment, while explaining why they’re not only the best solution, they’re economical too:

• Speed: The first thing hard drives have going for them is speed. You may have hundreds of gigabytes of data on your computer’s internal hard disk, but copying such large amounts of data can be extraordinarily time-consuming under the best of circumstances. Even fast optical drives and tape drives transfer data at a fraction of the speed of a slow hard drive. If you want to do more with your computer than watch it back up your data, you’ll appreciate the time savings a hard drive provides.

• Capacity: If you’re backing up to a medium with less capacity than your hard disk, sooner or later you’ll have to swap media. Even the newest Blu-ray media can’t store the entire contents of a moderately large hard disk on a single disc. Swapping media isn’t the worst thing in the world, but the more often you have to do so, the more of an aggravation backing up becomes. If, on the other hand, you use an external hard drive with sufficient capacity, you’ll never have to swap media—and you can allow your backups to run unattended at any time of the day or night.

• Random access: In addition to raw speed in copying files, hard drives offer the advantage of random access. (Tapes, by contrast, offer only linear access—the drive must fast-forward or rewind to get to any arbitrary piece of data.) Besides using space more efficiently, this means it takes no longer to restore files stored over a period of weeks than it does to restore files stored on a single date.

• Versatility: When you use a hard drive for backups, you can put both duplicates and versioned backups on the same device. You can (usually) boot from it, and even, in a pinch, use it as supplemental storage for other projects. Plus, using a hard drive keeps your optical drive (or other removable storage devices) free for installing software, burning DVDs, or other day-to-day tasks.

• Economy: As I write this, 1TB FireWire/USB combo drives can be found at retail for well under $150, and if you look online at discount stores and eBay auctions, you can find them for even less. (By comparison, when I wrote the first edition of this book in 2004, the going price for a 160 GB hard drive was close to $200.) That’s quite a bargain—especially when you factor in the recurring costs of optical media or tapes.

Further, how much is your time worth? Can you afford to spend an entire day restoring from a stack of CD-ROMs? If, instead, you could be up and running minutes after a drive failure, what would that be worth to you? Based on my own experience, I can say with conviction that an initial investment of a few hundred dollars pays for itself many times over when you consider the time and aggravation it saves in the long run.

Warning: If you have a large internal hard disk and far too little data to fill it, you may be tempted to partition it into several volumes and store backups on each one—instead of using separate physical drives. Although this is marginally better than not backing up at all, it’s still an incredibly bad idea. Hard drives usually don’t die one partition at a time. You could easily encounter a problem that makes it impossible to access any part of the disk, in which case your backups would be useless. And just like a second internal drive, a second partition is vulnerable to theft and damage that affects your entire computer.

Does size matter?

If you’re using a hard drive for backups, how large does it need to be? This seemingly tricky question has a relatively easy answer: as a rule of thumb, a destination volume should have between 1 and 1.5 times the capacity of the source volume. Sometimes one can comfortably store both a duplicate and several months’ worth of a versioned backup on a single disk the same size as the one being backed up—but you can check this with a little bit of math.

Except when backing up to a Time Capsule (in which case you’ll use its disk only for versioned backups and rely on a separate drive for duplicates), I advocate partitioning each backup disk into two volumes—one for a duplicate and one for versioned backups. It’s easy to figure out how much space you need for each, and then add the two amounts together to get a total disk size for the backup drive.

The Finder’s Get Info window for a hard disk. The number next to Used indicates the amount of data currently stored on the volume.Duplicates: For duplicates, you need a volume that will hold all the data on your disk—which may be much smaller than its actual capacity—and provide extra breathing room. To find out how much space your data currently occupies, select your hard drive’s icon in the Finder and choose File -> Get Info. The figure next to the word “Used” (shown on the right) is the amount of space the data currently occupies.

Assuming that you regularly add new files to your computer, you will want to leave a significant cushion to accommodate the files you’re likely to add during the next 6 to 12 months. If you do not have a good sense of the rate at which your data will grow, multiply the Used figure by 1.5, and then round up to the nearest gigabyte. (In this example, the volume “Leopard” would require at least 487GB for a duplicate.) In any case, your duplicate volume need never be larger than the total capacity of the disk you’re backing up.

Begin by determining the total space occupied by the data you plan to back up (again, use the Finder’s Get Info command), which could be your entire disk, or perhaps only your home folder if you perform a selective backup. Next, subtract the total size of any folders you intend to exclude (for example, ~/Music/iTunes/iTunes Music). Now multiply this total by 1.5. The resulting figure—let’s call it x—is the minimum amount of space you should allot for a versioned backup partition if you’re using compression. Without compression (and remember, if you’re using Time Machine, it doesn’t compress your backups), multiply x by two.

Note: If you’re backing up several Macs over a network to a single set of media, be sure to perform these calculations for each computer, and then add them together. Although some network backup software can save space by maintaining just one copy of a file that’s identical across multiple computers, you’ll be safer if you ignore that possibility and allow more breathing room.

I hasten to point out that these figures represent recommended minimums. They will enable you to back up your data comfortably today, but as your hard disk fills up, you want a backup disk that can keep up with it. So all things considered, you should buy a backup disk with a higher capacity than what you think you need right now. Given the rapidly falling prices of hard drives, and the decreasing differences between medium- and large-capacity models, it no longer makes sense to get a slightly smaller disk just to save a few dollars. As of mid-2009, desktop drives in the 500GB to 1.5TB range provide, on average, the best value (in gigabytes per dollar), and one or more of those drives will make the most sense for the majority of users.

Choosing a hard drive

Because so many different external hard drives exist, the choice can be daunting. Here’s my quick guide to what you need to know.

Interface: You can get hard drives with almost any combination of FireWire 400, FireWire 800, USB 2.0, and eSATA (external Serial ATA) interfaces. The theoretical speed with which these interfaces can transfer data goes in this order, from slowest to fastest: FireWire 400, USB 2.0, FireWire 800, and eSATA. However, in practice, USB 2.0 is nearly always much slower than either version of FireWire. FireWire 800 can be significantly faster than FireWire 400, but usually not twice as fast, as the number implies—you may see only a modest speed increase. Finally eSATA is unquestionably the fastest, but as of mid-2009, no Macs come with eSATA ports; you have to add them with a third-party PCI or PCI-X card (for Power Mac and Mac Pro models), a PC card (for PowerBooks), or an ExpressCard/34 (for some MacBook Pros). Even then, not all adapters support booting a Mac from an eSATA drive (check with the manufacturer if in doubt).

For most people, FireWire 800 is the best choice (assuming your Mac supports it), followed by FireWire 400 and USB. In any case, be sure to get a drive with at least one interface that matches what’s on your Macintosh. (For example, the MacBook Air has no FireWire ports—just USB 2.0.) If you can afford the slightly higher price, you won’t go wrong with a quad-interface drive, which has USB, both FireWire varieties, and eSATA.

One-touch backups: Maxtor sells OneTouch external hard drives with a button that enables you to launch software and execute a backup just by pressing it. I’d rather have my backups run automatically or on a schedule—one less button to press! (Note: Seagate purchased Maxtor in mid-2006, but the new combined company is keeping the two brands separate for the time being.)

Automatic backups: CMS Products’ ABSplus drives include software that performs a duplicate as soon as you plug in the drive. That’s great—but only part of what we want. I’d opt instead for the flexibility of standard backup software. Feel free to get an ABSplus, but plan to supply your own software, at least for archiving.

The first time you plug the drive into your Mac, you run the included setup program, which requires only a couple of clicks. Thereafter, the drive automatically backs up important files whenever it’s plugged in. No scheduling, configuration, or manual intervention is ever required, but if you need to restore files, you can do so by running the software included on the drive. Although that all sounds marvelous in that it’s incredibly easy to use, bear in mind that Clickfree products provide neither bootability nor versioned backups, my two key pillars of a solid backup strategy. So I suggest resisting the temptation and sticking with conventional drives.

Build-your-own: Numerous companies sell FireWire-, USB-, and/or eSATA-equipped cases into which you can place your own IDE or SATA drive mechanism. If you’re comfortable doing some minor tinkering and bargain hunting, you may be able to save a bit of money this way. (See the next item, also, for an alternative.)

Caseless connector kits: You don’t necessarily need a case to connect a bare drive mechanism to your Mac. Several companies offer adapters that connect various combinations of bare IDE or SATA drives directly to USB or FireWire ports. Although you’ll have to go without the additional protection and ventilation that a case provides (making them best only for short-term use), you can save money and space with one of these. Examples include:

WiebeTech’s line of DriveDock products, such as the UltraDock v4 (FireWire 800, FireWire 400, USB 2.0, and eSATA interfaces for any 3.5-inch IDE drive, or 2.5-inch or 3.5-inch SATA drive, $199)—other models have various other combinations of interfaces